Information on Mexico's middle Miocene mammal record improves understanding of the southern extent, makeup, and relationships of North American Tertiary faunas. The Hemingfordian–Barstovian combined assemblage (= HBCA) in Mexico records 6 orders, 17 families, and 35 genera, each represented probably by a single species known from sites in Baja California Norte and Sur (one each), Sonora (two), Aguascalientes (one), Oaxaca (three), and Chiapas (one). Barstovian diversity is nearly triple that of the Hemingfordian. The Barstovian Oaxacan subassemblage is by far the largest. This combined assemblage includes most of the orders and a little over a third of the families known to occur in North America for this interval. The better Tertiary post-Barstovian record does not include Clarendonian age sites, and has no representation in southern Mexico.

The HBCA consists mainly of herbivores and has very few carnivorans. The Barstovian equids from Southeastern Mexico include an ancestral merychippine coexisting with hipparionines and pliohippines. The coexistence of two derived lineages implies either repeated southward migration of equid species from temperate North America shortly after their origination or that some equid differentiation took place in tropical Middle America. Further work may clarify this issue. The HBCA shows strict North American affinities. Seven families and 16 genera have their southernmost occurrence in Mexico. The putative recent discovery in Peru of North American Clarendonian or older mammals may indicate that at least the southward migrating component of the Great American Faunal Interchange occurred earlier than currently thought. Three of the four families recorded in Peru occur in the Barstovian of Southeastern Mexico, thus lending support to this contention.